1. Field of the Invention
The present invention relates to a process for continuously drying wet coal to a preselected moisture content. More particularly, it relates to drying a stream of wet particulate coal in a fluidized bed wherein the energy for drying is supplied by partial combustion of the coal itself.
2. Description of the Prior Art
Lignite and subbituminous coals found in the western United States both have a low sulfur content which is desirable to utilities seeking to comply with increasingly strict air-quality standards. Offsetting this advantage, western coals have a high moisture content and resulting low BTU values. Most utilities in the eastern United States have boilers designed for burning eastern coal having a low moisture content. Conversion of these boilers to allow combustion of western coals is expensive and generally results in a reduced steam generating capacity. For these reasons, it is desirable to lower the moisture content of western coals thereby increasing their BTU value and making them compatible for use in boilers presently burning eastern coals.
An additional advantage of drying the coal prior to shipment is to decrease shipping costs because of the resulting weight reduction. The advantages to drying western coals are set forth in detail in the article entitled "Enriching Western Coals By Thermal Drying and Oil Treatment by Wegert and Jensen in Coal Age, May 1976.
The drying of particulate coal in a fluidized bed is well known in the prior at. Most often, heated combustion products are used to fluidize the bed and to provide the enthalpy necessary to dry the coal. U.S. Pat. No. 3,755,912 to Hamada et al describes a process wherein the off-gases from a coking oven at a temperature of approximately 250.degree. C. (482.degree. F.) are further heated to approximately 500.degree. C. (932.degree. F.) in a separate furnace and, thereafter, used to fluidize and dry a bed of coal. U.S. Pat. No. 3,190,627 to Goins reveals a fluidized bed dryer having a plurality of burners at the bottom thereof. The burners are gas-fired and provide combustion products directly to the fluidized bed overhead. The gases are in the temperature range from 1000.degree. F. to 3200.degree. F.
Several processes have utilized the combustion of coal to provide the heat necessary to dry the product coal. U.S. Pat. No. 3,896,557 to Seitzer et al provides for collection of coal fines above a fluidized drying bed and the burning of these fines in a separate combustion chamber. The resulting combustion products are used to fluidize the drying bed. Similarly, U.S. Pat. No. 2,638,684 to Jukkola describes a process wherein coal fines are collected above the fluidized bed and directed to the primary combustion zone. Jukkola describes a single vessel having both a combustion zone and a separate coal drying zone. Virtually complete combustion of the coal fines occurs in the combustion zone and the resulting combustion products are directed upward into the coal drying zone where wet coal feed is fluidized and dried.